A worm gear of this type is disclosed in DE-OS No. 2030260. This worm gear is preferably used for the conversion of rotary movement into linear movement. By providing a sufficiently large thread pitch, this worm gear can also be used conversely, namely to produce rotary motion by means of linear motion. In place of the single-thread embodiment, it is possible to provide a multi-thread embodiment whereby for the same load capacity, a steeper thread pitch can be produced. The thread may, for example, have a trapezoidal profile. Thereby a relatively large-surfaced thread flank having a large load capacity is attained. Under axial loading, it is desirable that the load be distributed over practically the entire thread length, i.e. the thread of the spindle and the thread of the nut must be manufactured with extremely small tolerances. In order to avoid manufacturing all of the thread dimensions with such extreme precision, the spindle and nut in many cases are provided with a small axial play, by means of which under pure axial loading a small gap is formed between the unloaded thread flanks. Under the opposite axial loading the gap is shifted to the other thread flanks of the outer and inner threads.
In the unloaded condition or under radial loading, on the other hand, the thread flanks of the inner thread are in contact with the thread flanks of the outer thread at corresponding circumferential positions along the thread. The known worm gear is designed so that in this case in the area of the loaded thread flanks a relatively large gap still exists between the crown surface of the outer thread and the groove bottom of the inner thread. Under normal loading conditions this known worm gear works without defect. For complex load distribution, however, the case can occur where the nut takes on an inclined position or tilts relative to the spindle. This limiting case has disadvantageous consequences for the functioning of the worm gear, particularly when rotation of the spindle is to be produced from linear movement of the nut or correspondingly when rotation of the nut is to be produced from linear movement of the spindle. As a result of the inclined position, the load-carrying surfaces are reduced to a minimum such that only an infinitesimal line or even a point of contact remains between the respective radially opposite and the respective axially opposite edge areas, i.e. the crown surfaces and the thread flanks, of the outer thread of the spindle and the inner thread of the nut. In nearly all cases the inclined position effects a jamming under small deformation of the contact surfaces. In this case relative rotation between the spindle and nut is, if it occurs at all, only made more difficult. In the foregoing example of conversion from linear to rotary motion, a self-locking occurs such that movement is no longer possible. The inclined position must first be corrected in order to ensure faultless functioning.